A printer, facsimile, photocopier, and a multi function peripheral (MFP) having these three functions or the like are used as an image forming device that forms an image by an electrophotographic process.
Such an image forming device provides a developing device, an exposing device, a transferring mechanism and a fusing device. The developing device forms a developer image on a surface of a photosensitive drum, which will be described below, by a developer. The exposing device exposes the photosensitive drum, which will be described below, and forms an electrostatic latent image, which will be described below, on the surface of the photosensitive drum. The transferring mechanism transfers the developer image formed on the surface of the photosensitive drum to a transfer medium. Additionally, the “transfer medium” means a medium on which the developer image described below is transferred, and means a recording medium such as paper and a transferring belt. The fusing device fuses the developer image transferred on the recording medium on the recording medium.
In the developing device, configuring elements such as a toner cartridge, a toner supply roller, a photosensitive drum, a charge roller, a developing roller, a developing blade, a cleaning blade are integrated. The developing device is configured in a detachable manner with respect to the image forming device. Some developing devices provide a detachable toner cartridge.
Some of the conventional image forming devices provide an elastic layer of the toner supply roller that is formed in foam structure having a number of apertures, which is referred to as a “cell,” so as to efficiently supply the toner to the developing roller and/or to efficiently clean (scrape) residual developing toner from the developing roller. See Japanese laid-open patent application publication number 2005-148664.
In the conventional image forming device, in order that the elastic layer of the developing roller is deformed into the elastic layer of the toner supply roller, the distance between the rotation axis of the developing roller and the rotation axis of the toner supply roller (hereafter referred to as an “interaxial distance”) is configured smaller than the sum of the radius of the developing roller and the radius of the toner supply roller. Namely, the conventional image forming device has a configuration of squeezing the developing roller and the toner supply roller in a smaller distance than the sum of the radius of the developing roller and the radius of the toner supply roller.
Such conventional image forming devices are configured to adjust a supplying-ability of the toner to the developing roller and a cleaning (or scraping)-ability of the residual developing toner from the developing roller by arbitrarily designing a deformation amount between the developing roller and the toner supply roller, the shape of the cell of the toner supply roller, the radius of the cell, the thickness of the cell wall, etc. Additionally, the “deformation amount” means an amount of how much a hard member (herein, the developing roller) deforms a soft member (herein, the toner supply roller) when the two rollers are squeezed in a frame shorter than the interaxial distance.
The conventional image forming device often provides a plurality of developing devices corresponding to colorization. However, when the conventional image forming device is designed, it is not considered that toner contained in each developing device is transformed due to heat from a fusing device that functions as a heat source so that a variation of adhering force occurs: a variation of a charging characteristics occurs depending on colors of the toners. Thereby, the conventional image forming device occasionally fails to properly scrape the residual developing toner in the developing device that is closely positioned to the fusing device as a heat source and/or in which highly charged toner is used due to the above variations.
Additionally, there is another problem that downsizing is prevented when the conventional image forming device is designed to suppress the failures of scraping the residual developing toner.
Namely, in order to suppress the failures of cleaning the residual developing toner in the conventional image forming device, the deformation amount between the developing roller and the toner supply roller of all of the developing devices and/or the deformation amount between the developing roller and the developing blade of all of the developing device etc. are increased in all of the developing devices. However, a large amount of load torque is required to rotate the developing roller and the toner supply roller of all of the developing devices for suppressing the above failures. Therefore, the conventional image forming device requires a high-output motor. As a result, the conventional image forming device is relatively large sized and is prevented from downsizing.